1. Field of the Invention
The invention relates generally to the field of photography, and particularly to a film cassette containing roll film. More particularly, the invention relates to a film cassette capable of propelling a non-protruding film leader to the exterior of the cassette shell.
2. Description of the Prior Art.
In conventional 35mm film manufacturers' cassettes, such as manufactured by Eastman Kodak Company and Fuji Photo Film Co. Ltd., the filmstrip is wound on a flanged spool which is rotatably supported within a cylindrical shell. A leading section of the filmstrip approximately 21/3 inches long, commonly referred to as a "film leader", protrudes from a light-trapped slit or mouth of the cassette shell. One end of the spool has a short axial extension which projects from the shell, enabling the spool to be turned by hand. If the spool is initially rotated in an unwinding direction, the film roll inside the shell will tend to expand radially since the inner end of the filmstrip is attached to the spool, and the fogged leader section protruding from the slit will remain stationary. The film roll can expand radially until a firm non-slipping relation is established between its outermost convolution and the inner curved wall of the shell. Once this non-slipping relation exists, there is a binding effect between the film roll and the shell which prevents further rotation of the spool in the unwinding direction. Thus, rotation of the spool in the unwinding direction cannot serve to advance the filmstrip out of the shell, and it is necessary in the typical 35mm camera to engage the protruding leader section to draw the filmstrip out of the shell.
A 35mm film cassette has been proposed which, unlike conventional film cassettes, can be operated to automatically advance the filmstrip out of the cassette shell by rotating the film spool in the unwinding direction. The film leader normally does not protrude from the cassette shell. Specifically, in U.S. Pat. No. 4,407,579, granted Oct. 4, 1983, there is disclosed a film cassette wherein the film spool is comprised of two spool pieces constructed to telescopically receive one another. Each spool piece includes a radially extending flange which is adapted to lie closely adjacent to but not in contact with one of the opposite sides of the film roll. A trailing end of the film roll is attached to one of the spool pieces, and a leading end of the roll rests against a cylindrical insert located between the two flanges and the cassette shell. The insert restricts the maximum diameter of the film roll to a value less than the diameter of the flanges, and it includes an integral stripper-guide located immediately ahead of a slot in the insert which leads to the light-trapped film passage opening of the cassette shell. To move the leading end of the film roll over the stripper-guide, into the slot, and through the light-trapped opening, to make it available outside the film cassette, one must manually grasp the opposite ends of the film spool and move the two spool pieces axially toward one another until the flanges are axially positioned in firm clamping relation with the opposite sides of the film roll. While maintaining this inwardly directed manual pressure against the two spool pieces, one then must rotate the film spool relative to the cassette shell in an unwinding direction. As a result, the leading end of the film roll will be rotated relative to the insert, causing it to be advanced over the stripper-guides into the slot, and through the light-trapped opening to the exterior of the cassette shell. The film cassette disclosed in U.S. Pat. No. 4,407,579 has several shortcomings. For one, it must be operated manually to advance the leading end of the film roll to the exterior of the cassette shell. This must be done before the film cassette is loaded into a camera. Another shortcoming is that to increase the gripping effect of the two flanges at the opposite sides of the film roll, the flanges include a friction-producing material such as rubber which is axially compressed against the opposite sides of the film roll. This significantly increases the manufacturing expense of the film cassette. Moreover, it is possible especially in high temperature and/or humidity conditions that the opposite sides of the film roll may adhere to the rubber, in which instance a film advance failure may occur.
Commonly assigned U.S. Pat. No. 4,846,418, granted July 11, 1989 discloses a film cassette wherein the leading end of a film roll coiled about a film spool is located within the cassette shell. The film spool is comprised of two spool pieces constructed to telescopically receive one another. Each spool piece includes a radially extending flange normally disposed adjacent one of the opposite sides of the film roll. When the film spool is initially rotated in a film unwinding direction, a rotation-responsive cam device coupling the two spool pieces automatically moves one spool piece axially toward the other piece to cause the respective flanges to firmly clamp the film roll at its opposite sides. Then, with continued rotation of the film spool in the same direction, the leading end of the film roll will be advanced over a stripper-guide which strips the leading end from between the flanges and guides it through a light-trapped opening in the cassette shell to the outside of the cassette shell. The two flanges are slightly concave-shaped, and several ribs located on an interior wall of the cassette shell protrude radially between the flanges to circumferentially confine the outermost convolution of the film roll in order to prevent that convolution from clock-springing beyond the periphery of each flange. When the flanges firmly clamp the film roll at its opposite sides, the slight concave shape of the flanges may operate to slightly transversely pinch the outermost convolution of the film roll. In any event, the leading end of the film roll will tend to slightly separate from the next-inward convolution of the film roll to facilitate movement of the leading end over the stripper-guide responsive to rotation of the film spool in the film unwinding direction.